It is known that optical fibers having a doped core in which the doping is carried out with the use of particular substances, such as rare earth ions, have stimulated emission features and are suitable for use in optical amplifiers in optical fiber telecommunication lines for civil telephony.
Such amplifiers are referred to in the European patent application No. 90112920.5, which corresponds to said U.S. application Ser. No. 552,918 of the assignee of this application.
By "fiber optical amplifiers", also referred to as "active fiber optical amplifiers", is meant amplifiers in which the optical transmission signal is amplified as such, while keeping its optical form without a conversion of the same to another form, such as, for example, an electronic conversion in which there is conversion of the optical signals to electrical signals amplification of the electrical signals and a conversion of the electrical signals to the optical form. In fiber optical amplifiers, the amplifying element consists of a portion of optical fiber of the type described having a predetermined length, connected in series between two lengths of optical line fiber and provided with feeding means to feed the optical pumping signal.
Fiber optical amplifiers have particular advantages for use in telecommunication lines as they offer high gains when they are utilized as line amplifiers, and such gains can be brought to the desired value by suitably selecting the active fiber length and/or dopant content or, should they be used as power amplifiers, they offer a high amplification efficiency.
Particularly detrimental to such amplifiers are the signal reflections which occur at the ends of the fiber itself.
From Japanese patents 52-155901 and 63-219186 and from "ELECTRONICS LETTERS", vol. 24, no. 1, 7th Jan. 1988, pages 36-38, it is known that, in a laser or optical semiconductor amplifier, there is the risk of instability and arising of oscillations due to the reflections at the amplifier ends.
In the above patents and article, in order to eliminate these reflections, it is broadly taught to couple an optical isolator to the semiconductor laser, which prevents the light reflected by the coupling surfaces between the line fibers and these devices from reaching the lasers themselves.
In an active-fiber amplifier, no interface surfaces are present between the line fibers and the amplifier because the line fibers are directly welded to the active fiber of the amplifier. Therefore, the reflection phenomena are not generally expected.
However, it has been discovered that in an active-fiber amplifier, in the absence of means limiting reflections toward the active fiber, it is impossible to reach high amplification gains due to the occurrence of noise of the interferometric type as a result of beats between the direct signal and reflected signals in the line fibers themselves and, at all events, directed toward the active fiber. The presence of interferometric noise is of little importance in a semiconductor amplifier which has low gains and small construction sizes, whereas it becomes particularly important in an active-fiber amplifier capable of reaching very high gains and having an active fiber of significant length, generally in the range of some tens of meters and much greater than the coherence distance of the signal generating laser.
Therefore, in an amplifier of the active core fiber type, the problem arises of protecting the active fiber with respect to such noise source and keeping each form of reflection toward the active fiber itself below critical values so as not to jeopardize the transmission quality while maintaining high values of the amplification gain.
The above-cited European patent application No. 90112920.5 teaches the introduction of optical isolators in fiber optical amplifiers, said isolators having a reflectivity limited to below a critical value.